The present invention relates to bicycles, and is particularly concerned with bicycles having a planetary gear-change mechanism contained in the hub of the driven wheel.
Hub mounted gearboxes are well known in the bicycle art, and are conventionally based on the widely understood, planetary gearbox principle. Planetary-geared hubs for bicycles having two, three, four, five, seven, eight, and even fourteen speed ratios are currently available commercially.
In a conventional three-speed hub-mounted gearbox, a central spindle is provided which is rigidly attachable at both its ends to the bicycle frame. A sun gear is non-rotatably mounted to the spindle, and engages planet gears mounted in a planet carrier rotatably carried by the spindle. A drive sprocket, to engage the bicycle chain, rotates coaxially with the spindle.
An annulus gear is engaged with the planet gears, and is movable to engage and rotate with either the drive sprocket or with the outer casing of the hub, to which the spokes of the wheel are attached. The planet carrier is likewise movable to engage and rotate with either the drive sprocket or the hub.
In the “low” gear position, the drive sprocket and annulus rotate together, and the planet carrier rotates with the hub, so that the wheel rotates more slowly than the drive sprocket.
In the “middle” gear position, the drive sprocket engages the annulus and the hub likewise engages the annulus, so that the wheel rotates at the same speed as the drive sprocket.
In the “high” gear position, the drive sprocket engages the planet carrier and the annulus engages the hub, so that the wheel rotates faster than the drive sprocket.
The present invention does not concern the inner workings of the hub gearbox, beyond the considerations that the bicycle wheel is mounted to the outer casing of the gearbox, and the central spindle must be held against rotation relative to the bicycle frame in order to transmit torque from the drive sprocket to the outer casing of the gearbox and thence to the wheel.
The main advantage of a planetary-geared hub is that the gears are compactly sealed within the hub outer casing. Because the gears are protected from water and debris, planetary-geared hubs require low maintenance.
Another advantage is that there is no requirement for a rear derailleur mechanism to shift gears. This simplifies the bicycle chain drive because the chain path does not move relative to the bicycle frame as the rider changes gear, as is the case with conventional multi-sprocket derailleur arrangements. Since there is a fixed chain path, a chain cover can be fitted to the bicycle to further protect the bicycle chain drive from dirt. This chain cover also protects the rider's leg or clothing from soiling by chain lubricants.
There are, however, drawbacks to the current state of the art in planetary-geared hubs. Because planetary-geared hubs are usually designed with a long central spindle whose ends are adapted to be mounted to the bicycle frame on either side of the rear bicycle wheel, then in order to remove the tyre from the wheel, for example if the tyre is punctured, the entire rear wheel—including the planetary-geared hub—must be removed from the bicycle frame in order to repair the flat tyre. This can be extremely time consuming.
If the bicycle has been fitted with a rear chain drive cover, even more time is required to remove the rear wheel. First the cover must be removed, and then the planetary-geared hub axle nuts must be loosened. The bicycle drive chain can then be removed and consequently, the planetary-geared hub, including the rear bicycle wheel, can be removed and the tyre can then be repaired.
Another drawback to current planetary-geared hubs is that if the rear wheel needs to be removed for transporting the bicycle in a car or for storing the bicycle in a compact space, the same time-consuming procedure must be followed.
It has been proposed, in a bicycle referred to as an “Entertainment Vehicle” and exhibited by Sony Corporation in Japan in 2000, to mount a planetary-geared hub to a bicycle frame having a single rear arm extending on only one side of the rear bicycle wheel. Such a mounting allows the tyre to be removed without the need to remove either the planetary geared hub or the wheel from the bicycle frame. The Entertainment Vehicle has a solid rear arm, in order to achieve the required rigidity and strength, and an entirely exposed rear chain drive comprising a drive sprocket attached to pedals mounted to the bicycle frame, a driven sprocket at the rear hub, and a drive chain linking the sprockets.